Application of anomalous X-ray diffraction on mixed-valence uranium oxides

Abstract

Abstract and presentation of a featured oral presentation at the Uranium Science Conference 2023 (5-7 July, 2023), Manchester, UK. ---Abstract--- The binary uranium-oxygen system contains numerous single-valence and mixed-valence uranium oxide compounds, spanning over a wide phase field which is defined by the oxygen-to-uranium ratio (O/U) and temperature. Uranium dioxide (UO2), the principal component of commercial nuclear fuel, is nominally associated to the tetravalent uranium state. It is, however, quite susceptible to oxidation and already at room temperature excess oxygen can be accommodated near the unoccupied octahedral sites in its fluorite-type crystal structure. To maintain charge neutrality local changes in the uranium chemical state are induced, resulting in a mixed-valence charge character. At elevated temperatures (above 300 °C) an anion-excess fluorite crystal structure is maintained up to the average composition UO2.25. However, at lower temperatures distinct structural phase transformations occur upon exceeding the limit UO2.03 2.05, resulting in the occurrence of mixed-valence compounds at or near specific O/U ratio values: U4O9-y (O/U ≈ 2.25), U3O7 (O/U = 2.333) and U3O8 (O/U = 2.667). In the context of nuclear fuel degradation under storage conditions, and in accident scenarios, significant research efforts have been dedicated to an improved, mechanistic understanding of the crystallographic phase relations between these uranium oxide compounds, and the associated evolution in the uranium chemical state [1-3]. While both properties are entangled with each other, most of the commonly employed theoretical and experimental techniques allow to probe one but are more indirect towards to other (e.g. diffraction and spectroscopy) [4,5]. Both properties can, however, be detected in harmony by performing X-ray diffraction in an energy spectrum around a core-level absorption edge, so-called diffraction anomalous fine structure (DAFS). We have recently applied this unique experimental method, relatively unknown in the field of actinide science, on a variety of uranium oxide compounds. In this contribution we will report on the obtained results, using newly developed data analysis approaches in Python and full-pattern refinement treatments. [1] E. De Bona, et al., Inorganic Chemistry, 61 (2022) 1843.[2] J.M. Elorrieta, et al., Physical Chemistry Chemical Physics, 24 (2022) 28394.[3] K.O. Kvashnina, et al., Physical Review Letters, 111 (2013) 253002.[4] G. Leinders, et al., Inorganic Chemistry, 59 (2020) 4576[5] G. Leinders, et al., Inorganic Chemistry, 60 (2021) 10550.